1. Field of the Invention
The present invention relates generally to the field of quality control for formation fluid sampling and in particular to the determination of permeability and mobility versus time to provide an indication as to whether a formation sample is in a single phase state, experiencing laminar flow and low filtrate contamination, to ensure acquisition of a single phase sample of optimal purity and in the same condition as it existed in the formation by applying formation rate analysis during pumping of a sample from a formation. The method and apparatus also provide for detection of pumping problems (correlation coefficient for pressure versus formation flow rate) and to the matching of an optimal pumping rate to the ability of the formation to produce (mobility, compressibility).
2. Summary of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations. Modern directional drilling systems generally employ a drill string having a bottom hole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or by rotating the drill string. A number of down hole devices placed in close proximity to the drill bit measure certain down hole operating parameters associated with the drill string. Such devices typically include sensors for measuring down hole temperature and pressure, azimuth and inclination measuring devices and a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional down-hole instruments, known as logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine the formation geology and formation fluid conditions during the drilling operations.
Commercial development of hydrocarbon fields requires significant amounts of capital. Before field development begins, operators desire to have as much data as possible in order to evaluate the reservoir for commercial viability. Despite the advances in data acquisition during drilling using the MWD systems, it is often necessary to conduct further testing of the hydrocarbon reservoirs in order to obtain additional data. Therefore, after the well has been drilled, the hydrocarbon zones are often tested with other test equipment.
One type of post-drilling test involves producing fluid from the reservoir, shutting-in the well, collecting samples with a probe or dual packers, reducing pressure in a test volume and allowing the pressure to build-up to a static level. This sequence may be repeated several times at several different depths or point within a single reservoir and/or at several different reservoirs within a given borehole. One of the important aspects of the data collected during such a test is the pressure build-up information gathered after drawing the pressure down. From these data, information can be derived as to permeability, and size of the reservoir. Further, actual samples of the reservoir fluid must be obtained, and these samples must be tested to gather Pressure-Volume-Temperature and fluid properties such as density, viscosity and composition.
In order to perform these important tests, some systems require retrieval of the drill string from the borehole. Thereafter, a different tool, designed for the testing, is run into the borehole. A wireline is often used to lower the test tool into the borehole. The test tool sometimes utilizes packers for isolating the reservoir. Numerous communication devices have been designed which provide for manipulation of the test assembly, or alternatively, provide for data transmission from the test assembly. Some of those designs include mud-pulse telemetry to or from a down hole microprocessor located within, or associated with the test assembly. Alternatively, a wire line can be lowered from the surface, into a landing receptacle located within a test assembly, establishing electrical signal communication between the surface and the test assembly. Regardless of the type of test equipment currently used, and regardless of the type of communication system used, the amount of time and money required for retrieving the drill string and running a second test rig into the hole is significant. Further, if the hole is highly deviated, a wire line can not be used to perform the testing, because the test tool may not enter the hole deep enough to reach the desired formation.
An apparatus and method for measuring formation pressure and permeability is described in U.S. Pat. No. 5,233,866 issued to Robert Desbrandes, hereinafter the '866 patent. FIG. 1 is a reproduction of a FIG. from the '866 patent that shows a draw down test method for determining formation pressure and permeability. Referring to FIG. 1, the method includes reducing pressure in a flow line that is in fluid communication with a borehole wall. In Step 2, a piston is used to increase the flow line volume thereby decreasing the flow line pressure. The rate of pressure decrease is such that formation fluid entering the flow line combines with fluid leaving the flow line to create a substantially linear pressure decrease. A “best straight line fit” is used to define a straight-line reference for a predetermined acceptable deviation determination. The acceptable deviation shown is 2σ from the straight line. Once the straight-line reference is determined, the volume increase is maintained at a steady rate. At a time t1, the pressure exceeds the 2σ limit and it is assumed that the flow line pressure being below the formation pressure causes the deviation. At t1, the draw down is discontinued and the pressure is allowed to stabilize in Step 3. At t2, another draw down cycle is started which may include using a new straight-line reference. The draw down cycle is repeated until the flow line stabilizes at a pressure twice. Step 5 starts at t4 and shows a final draw down cycle for determining permeability of the formation. Step 5 ends at t5 when the flow line pressure builds up to the borehole pressure Pm. With the flow line pressure equalized to the borehole pressure, the chance of sticking the tool is reduced. The tool can then be moved to a new test location or removed from the borehole.
A drawback of the '866 patent is that the time required for testing is too long due to stabilization time during the “mini-buildup cycles.” In the case of a low permeability formation, the stabilization may take from tens of minutes to even days before stabilization occurs. One or more cycles following the first cycle only compound the time problem.
Whether using wire line or MWD, known formation pressure and permeability measurement systems measure pressure by drawing down the pressure of a portion of the borehole to a point below the expected formation pressure in one step to a predetermined point well below the expected formation pressure or continuing the draw down at an established rate until the formation fluid entering the tool stabilizes the tool pressure. Then the pressure is allowed to rise and stabilize by stopping the draw down. The draw down cycle may be repeated to ensure a valid formation pressure is being measured, and in some cases lost or corrupted data require retest. This is a time-consuming measurement process.
U.S. Pat. No. 6,609,568 teaches a formation rate analysis (FRA) apparatus and method that addresses some of the drawbacks described above by utilizing a closed-loop apparatus and method to perform formation pressure and permeability tests more quickly than the devices and methods described above. With quicker formation testing, more tests providing actual pressures and permeability may be provided to enhance well operation efficiency and safety. U.S. Pat. No. 6.609.568 provides an apparatus and method capable of creating a test volume within a borehole, and incrementally decreasing the pressure within the test volume at a variable rate to allow periodic measurements of pressure as the test volume pressure decreases. Adjustments to the rate of decrease are made before the pressure stabilizes thereby eliminating the need for multiple cycles. This incremental draw down apparatus and method will significantly reduce overall measurement time, thereby increasing drilling efficiency and safety.
There is a need for determining fluid mobility while pumping in order to provide quality control and confidence during sampling. There is a need to determine the formation fluid quality and constitution. There is also a need to detect problems during pumping associated with loss of packer seal, sanding and sample fluid going to two-phase.